1. Field of the Invention
This invention relates generally to valve actuators having "biased to safety position" closure systems, and more specifically, concerns fluid operated and fluid returned valve actuators with a single pressurized fluid source being controllably utilized to return one or a plurality of valves to the "safe" position thereof responsive to a predetermined control sequence.
2. Description of the Prior Art
Valve and valve actuator mechanisms that are designed for "fail-safe" operation of, for example, gate valves, typically comprise a valve operating stem for moving a gate member linearly between its open and closed positions within a valve body. The valve operator stem extends through a cylinder with a piston connected to the actuator stem being linearly moveable within the cylinder by a pressurized fluid medium such as fluid entering the cylinder on the supply side of the piston. The term "fluid" as used in the specification and claims is intended to include a hydraulic fluid and a compressible gas fluid. Actuating fluid from the return side of the piston is typically exhausted to a storage receptacle or accumulator (alternatively to the sea for subsea applications) as the piston is driven by the pressure of fluid on its supply side. As the piston is being moved by supply pressure, thus typically opening the valve, a preloaded compression spring acting on the actuator stem and opposing the force of supply pressure is further compressed as the actuator piston is moved by supply pressure.
For fail-safe closure of the valve, the pressure of the supply fluid is vented to dissipate the pressure-induced valve opening force on the actuator stem, thus allowing the force of the compression spring to drive the actuator stem outward in relation to the valve body, thus moving the gate of the valve mechanism to its closed position. Notably, valve body pressure acting on the stem typically assists the spring in moving the valve gate to its "safe" position (i.e., "closed" in the foregoing discussion). To accomplish this purpose, what was "supply" fluid during the valve opening operation must be exhausted from the supply side of the piston to accommodate the spring/pressure induced valve closing function. The "supply" fluid must either be moved in a reverse flow direction within the supply line or it may be vented by appropriate control as the "return" fluid is drawn in from a storage accumulator or other source. In the alternative, and being the preferred the "supply" fluid can be routed via a control valve to the return side of the piston, coincidentally displacing "supply" fluid and replacing "return" fluid. An accumulator is needed if the volumes of the supply and return sides of the actuator are different. Systems similar to those described above are typically also used for operation of other types of valves (e.g., ball, plug, butterfly, etc.).
Currently available spring-returned valve actuator mechanisms, especially those designed for deep water submerged applications, incorporate return springs that are very large, and require that even larger "valve actuator housings" be provided to protect them. The resulting fail-safe actuators are therefore large and heavy, are consequently quite costly, and result in correspondingly large and expensive systems built up using these components. It is desirable, therefore, to provide a method and apparatus for "fail-safe" valve closure for subsea and other valves that does not require that each valve actuator be equipped with a return dedicated spring. It is also desirable to provide a system incorporating multiple fail-safe valve and valve actuator assemblies wherein a single fluid pressure source is available for selective closure of one or more or all of the valve mechanisms in a system responsive to a predetermined condition or responsive to selective control.